We don't as yet know very much about black holes, but one of the
things we do know is that it's not a good idea to get
too close to one of them! Their powerful gravitational pull can rip
apart anything that passes nearby. Yet a star may have
survived such a close encounter, an encounter that was recently
observed by LJMU's David Bersier and colleagues using the Liverpool
Telescope.

An artist's impression of the passage of a
star close to a black hole, causing a a Tidal Disruption Event (image
courtesy CXO/NASA).

Light curves showing how the brightness
(magnitude) of the stellar debris steadily changes with time during
the star's encounter with the black hole. The event was observed
over a considerable period of time in a variety of imaging filters.
The u, g, r, i and z-band data were secured at the LT; the other data
were observed by Swift. (MJD is the Modified Julian Date.)

Only a few such stellar disruptions have been seen before. Close
encounters are thought to be rare, and to date their discovery has
largely been by accident. In order to catch such an uncommon event,
astronomers need to look at a large fraction of the sky, and look
often. This is what the
All-Sky Automated Survey for Supernovae (ASAS-SN, pronounced
"assassin") is designed to do. Its six small telescopes - four in
Hawaii and two in Chile - scan the sky every night, looking for
variable sources, transient objects, and sudden outbursts. ASAS-SN
discoveries often trigger rapid follow-up observations on larger
telescopes, particularly robotic facilities like the LT.

On January 25, 2014, an otherwise anonymous galaxy located a mere
650 million light years away in the constellation that contains the
"Big Dipper", looked significantly brighter than usual. This object,
nicknamed ASASSN-14ae, was initially thought to be a supernova, the
explosion of a massive star, albeit an unusual one.

Several telescopes, including NASA's Swift observatory
and the Liverpool Telescope, were immediately used to obtain more
data. PhD student Thomas Holoien of Ohio State University led the
effort and coordinated the observing campaign.

As the story unfolded it became clear that ASASSN-14ae was not a
supernova, but was instead something entirely different: a Tidal
Disruption Event, or TDE. Such an event is believed to occur
when a star gets a little too close to a black hole, an object with a
mass several million times that of our Sun. Luckily, the star in
this case seems to have survived the encounter, with only a small
chunk of matter being ripped off!

The amount of energy released during the event allowed researchers
to calculate that only one thousandth of the mass of our sun - about
the mass of the planet Jupiter - had been sucked into the black hole.

Light curves, showing how the brightness of the debris ripped from the
star varied during the encounter, are shown to the right. As the
debris falls towards the black hole it settles into an "accretion
disk", where it gets hot and thus shines. The steady decline in
brightness of this material, seen over a period of many weeks with
Swift and the LT, matches what is expected of a TDE.

The Liverpool Telescope is the perfect machine to follow an event
such as this. Although Holoien and his team needed access to a
telescope for only ten minutes or so each night, observations were
needed over a long period of time. The fact that the LT is entirely
computer-controlled means that the observations could be scheduled
remotely and all in one go: a very lengthy stay at an overseas
observatory was thus not required. This is perhaps a less romantic
way of observing, but is none-the-less a lot more efficient.

Monitoring the whole night sky every other night, the ASAS-SN survey
has a good chance of detecting more of these events, and perhaps even
more exotic cosmic catastrophes that we haven't thought of yet! In the
meantime, the LT will be ready and waiting to react to ASAS-SN
triggers and secure the observations needed to better understand these
remarkable cosmic phenomenon.

The observations described here have recently been published in
volume 445 of the Monthly Notices of the Royal Astronomical Society by
Holoien, Bersier and their collaborators. A copy of the article is
available
here.